:: Section 8
Curvilinear Motion and Stokes Number
II. Stokes Number
Curvilinear motion is characterized by a dimensionless number called the Stokes number (Stk). It is the ratio of the stopping distance of a particle to a characteristic dimension of the obstacle. For example, for a flow of diameter dc making a 90° turn, perpendicular to a cylinder the Stokes number is
where U is the flow velocity, for Rep < 1.0.
When Stk >> 1, particles continue moving in a straight line when the gas turns; when Stk << 1, particles follow the gas streamlines perfectly and make the turn. So Stokes number is the ratio of a particle's "persistence" to the size of the obstacle and it is used to characterize inertial impaction, which is described in the next section.